A) Field of the Invention
The present invention relates to a phosphor, and more particularly, to a phosphor suitable for an illumination light emitting device using a light emitting diode, its manufacture method, and a light emitting device using a phosphor.
B) Description of the Related Art
With the development of blue light emitting diodes, white light sources using light emitting diodes are used widely. White light sources using light emitting diodes can be used as portable light sources, back lights for liquid crystal displays, and illumination light sources such as car headlights. Illumination light sources using light emitting diodes are becoming important along with display light emitting diodes. An illumination light source using a light emitting diode emits white light by using a combination of a blue light emitting diode emitting light at a wavelength of, e.g. about 460 nm, and a yellow phosphor such as yttrium aluminium garnet with Eu activator (YAG:Eu).
Since red light components of this light source are small, color rendering and color reproducibility are insufficient. For example, there is a tendency of insufficient color reproducibility in the case of a back light of a liquid crystal display. As a method of improving color reproducibility, there is a method of emitting lights of three colors, e.g., red, green, and blue, instead of light of two colors, blue and yellow.
International Publication No. WO2002/091487 proposes a white light source having high color rendering using a combination of a blue light emitting diode, YAG yellow phosphors and Eu complex red phosphors.
International Publication No. WO000/33390 (National Publication N0. 2002-531956) proposes a combination of a blue light emitting diode, green phosphors and red phosphors to emit white light having high color rendering, with light of three primary color wavelengths.
Phosphors are thus used to generate white light from a single kind of light emitting diode. Certain phosphors like sulfides lose their luminous intensity (luminance) when they are exposed to humidity. Several countermeasures against deterioration of phosphor properties due to humidity have been proposed.
Japanese Patent Laid-open Publication No. HEI-4-230996 introduces techniques of forming an oxide coating film on sulfide phosphor particles through fluidized-bed CVD, and points out that exposure of the phosphor particles to a high temperature may lower the luminous intensity. It proposes phosphor particles having a high resistance against humidity, the phosphor particles being covered with a very thin (thickness being 0.1 μm to 3 μm, preferably 0.1 μm to 0.5 μm) oxide coating film by conducting hydrolysis reaction of oxide precursor at 25° C. to 170° C. on the surface of phosphor particles.
International Publication No. WO00/69986 (National Publication No. 2002-544365) proposes a method of forming a coating film on particles of micrometer size by mixing inorganic particles of a micrometer size with coating particles of nanometer size and firing the mixture